1. Field of the Invention
The present invention relates to an illumination optical system, particularly in detail to an illumination optical system which passes illumination light emitted from a laser through an optical integrator and uniformizes its intensity distribution.
Moreover, the present invention relates to an exposure method and device which exposes a photosensitive material by irradiating illumination light, modulated after being emitted from an illumination optical system as described above, onto the photosensitive material.
2. Description of the Related Art
Exposure devices designed to illuminate a two-dimensional spatial light modulator, such as a liquid crystal display (LCD) and a digital micromirror device (DMD), with light from a light source and light modulated by this spatial light modulator to expose photosensitive material, are known. Devices of this type are required to illuminate the spatial light modulator uniformly. Thus, an optical integrator is incorporated in an illumination optical system. As described above, the optical integrator which uniformizes the intensity distribution of illumination light is generally used in projectors and the like besides the exposure device.
The optical integrator is one which resolves and uniformizes a correlation (intensity distribution) between intensities and positions by dividing luminous flux and combining the luminous flux again after having passed through different paths. The optical integrator is classified broadly into two types, depending on differences in methods for dividing the luminous flux. One is a fly-eye type which divides a luminous flux spatially by use of a lens array (fly-eye lens) in which a plurality of lenses are placed two-dimensionally. The other is a rod type which performs angular division of a luminous flux by multipath reflection by using a glass rod or a hollow rod with a mirrored inner surface. Note that the lens shape of the fly-eye type optical integrator and the shape of the cross-section in the rod-type optical integrator are made similar to the shape of a spatial light modulator to be illuminated. Thus, even if the shape of the light source and the shape of the spatial light modulator are different from each other, efficient illumination is possible.
Japanese Unexamined Patent Publication No. 9 (1997)-68667 discloses an example of an illumination optical system using an optical integrator as described above.
Note that, in an optical device such as a projector to which the illumination optical system using the foregoing optical integrator is applied, conventionally, a fly-eye lens with an aspect ratio of approximately 4:5 and a lens cell size of approximately 5 to 10 mm is used.
In the conventional illumination optical system using the foregoing optical integrator, a light source having a large Etendue (to be described later), such as a discharge lamp, had been used. Therefore, when illumination efficiency was sought to be improved, it was necessary to increase a numerical aperture NA (hereinafter, referred to as “illumination NA”) on the side of an object to be illuminated. In the case that the conventional illumination optical system was applied to an exposure device in which, unlike a magnifying projecting device, photosensitive materials were exposed by light magnified at a 1:1 magnification ratio or less, the focal depth was too small, thereby deteriorating image quality. On the other hand, in the case that the illumination NA was decreased to increase the focal depth, the overall length of the optical system tends to be long. Thus, a problem has been recognized that an exposure device and the like to which this illumination optical system is applied has inevitably grown in size, and that physical layout of such a device is difficult.